1. Field of the Invention
This invention relates generally to an optical modulator for combining RF signals and, more particularly, to an optical phase modulation system including a single optical source and a series of optical phase modulators each receiving a separate RF signal, where the phase modulators operate to combine the RF signals.
2. Discussion of the Related Art
Various systems exist that require the need to combine two or more RF signals. For example, phased array antennas that are used to increase signal strength reception at certain frequencies require that the RF signal received by each antenna element in the array be combined in phase with each other. Various RF signal combiners are known in the art for providing RF signal combination for these types of systems. One known RF signal combiner is the Wilkinson power combiner that combines the RF signals in stages of two signals. The Wilkinson power combiner suffers from the drawback that the number of RF inputs must be known or the combiner will suffer from efficiency losses. Also, the Wilkinson power combiner operates completely in the RF domain, thus making it not applicable for those systems that make use of the optical domain for signal processing.
With the increased use of optical systems to increase signal speed and information transmission, it is becoming increasingly more important to provide RF combining in the optical domain. Various RF signal combiners that employ RF modulation of an optical signal can be used. For example, wavelength division multiplexing (WDM) of unique wavelengths of light has been used to separately modulate several RF signals. In WDM, separately modulated light beams, each having a unique wavelength, are multiplexed into a single fiber or optical waveguide. A single photodetector is then used to both demodulate the individual intensity modulated laser wavelengths and simultaneously and coherently combine them.
In a different optical design, intensity modulation of a single laser beam by the multiple RF signals can be done to provide the RF combining. However, intensity modulating a light beam with an RF signal creates distortion, making the modulation power inefficient. Also, intensity modulation requires excessive laser power to give high dynamic range for phased array antenna applications.
The WDM technique described above requires several lasers operating at different wavelengths. Intensity modulation schemes suffer from distortions. It is desirable to reduce the complexity of an RF combiner by employing a single laser source at a single frequency, and that does not require intensity modulation of the light. It is therefore an object of the present invention to provide such an RF combiner.
In accordance with the teachings of the present invention, an RF combiner is disclosed that combines a plurality of RF signals in the optical domain. The combiner includes a single optical source that generates an optical beam. The optical beam is directed through a series of optical modulators, such as optical phase modulators. Each modulator is responsive to an RF signal that is to be combined with other RF signals. Each modulator modulates the optical signal with the RF signal so that the modulations combine in an additive manner. A single optical phase demodulator is used to demodulate the composite phase modulated optical signal to generate the combined RF signal. Suitable delay devices can be provided between the optical modulators, or the RF signals can be matched, so that the RF signals combine in phase.
Additional objects, advantages and features of the present invention will become apparent to those skilled in the art from the following discussion and the accompanying drawings and claims.